A. Field of the Invention
The present invention relates to digital printing apparatus and methods, and more particularly to lithographic printing members for use with laser-discharge imaging devices.
B. Description of the Related Art
U.S. Pat. No. 5,339,737, the entire disclosure of which is hereby incorporated by reference, discloses a variety of lithographic plate configurations for use with imaging apparatus that operate by laser discharge. These include "wet" plates that utilize fountain solution during printing, and "dry" plates to which ink is applied directly.
All of the disclosed plate constructions incorporate materials that enhance the ablative efficiency of the laser beam. This avoids a shortcoming characteristic of prior systems, which employ plate substances that do not heat rapidly or absorb significant amounts of radiation and, consequently, do not ablate (i.e., decompose into gases and volatile fragments) unless they are irradiated for relatively long intervals and/or receive high-power pulses. The disclosed plate materials are all solid and durable, preferably of polymeric composition, enabling them to withstand the rigors of commercial printing and exhibit adequate useful lifespans.
In one disclosed embodiment, the plate construction includes a first layer and a substrate underlying the first layer, the substrate being characterized by efficient absorption of infrared ("IR") radiation, and the first layer and substrate having different affinities for ink or an ink-abhesive fluid. Laser radiation is absorbed by the substrate, and ablates the substrate surface in contact with the first layer; this action disrupts the anchorage of the substrate to the overlying first layer, which is then easily removed at the points of exposure. The result of removal is an image spot whose affinity for ink or the ink-abhesive fluid differs from that of the unexposed first layer.
In a variation of this embodiment, the first layer, rather than the substrate, absorbs IR radiation. In this case the substrate serves a support function and provides contrasting affinity characteristics.
In both of these two-ply plate types, a single layer serves two separate functions, namely, absorption of IR radiation and interaction with ink or an ink-abhesive fluid. In a second embodiment, these functions are performed by two separate layers. The first, topmost layer is chosen for its affinity for (or repulsion of) ink or an ink-abhesive fluid. Underlying the first layer is a second layer, which absorbs IR radiation. A strong, durable substrate underlies the second layer, and is characterized by an affinity for (or repulsion of) ink or an ink-abhesive fluid opposite to that of the first layer. Exposure of the plate to a laser pulse ablates the absorbing second layer, weakening the topmost layer as well. As a result of ablation of the second layer, the weakened surface layer is no longer anchored to an underlying layer, and is easily removed. The disrupted topmost layer (and any debris remaining from destruction of the absorptive second layer) is removed in a post-imaging cleaning step. This, once again, creates an image spot having an affinity for ink or an ink-abhesive fluid differing from that of the unexposed first layer.
An alternative to the foregoing constructions that provides improved performance in some circumstances is disclosed in U.S. Pat. No. 5,353,705 and hereby incorporated by reference. The '705 patent introduces a "secondary" ablation layer that volatilizes in response to heat generated by ablation of one or more overlying layers. In a typical construction, a radiation-absorbing layer underlies a surface coating chosen for its interaction with ink and/or fountain solution. The secondary ablation layer is located beneath the absorbing layer, and may be anchored to a substrate having superior mechanical properties. It may be preferable in some instances to introduce an additional layer between the secondary ablation layer and the substrate to enhance adhesion therebetween.
Most plate constructions currently in use are imaged by means of imagewise photoexposure, followed by standard photochemical development. A recent variation of this approach, exemplified by the Polychrome CTX material, utilizes constructions based on a substrate, a layer of photohardenable material, and a surface layer that contains conventional silver halide grains. Imagewise exposure of the first layer (e.g., by an imaging laser) to radiation to which it, but not the photopolymer is sensitive, followed by chemical development, results in a mask that bears the image pattern and overlies the as-yet unexposed photopolymer. The construction is then subjected to radiation to which the photopolymer is sensitive. The exposed portions of the mask prevent passage of actinic radiation to the photopolymer, while radiation passes freely through unexposed regions, resulting in an imagewise exposure of the photopolymer that is negative with respect to the initial mask exposure, and which anchors the photopolymer to the substrate. The mask and unexposed photopolymer are then removed. See, e.g., What's New(s) in Graphic Communications, Sept.-Oct. 1993, p. 4.
A variant of this approach to imaging is described in U.S. Pat. No. 5,102,756, which discusses plates that include a base layer, a layer of photohardenable material, and a surface layer of photosensitive marking material containing particles that migrate in response to light and electricity. The surface is exposed to an imagewise pattern of light under conditions that cause particle migration, rendering an otherwise opaque layer largely transparent. The construction is then exposed to radiation that cures the photohardenable material. That radiation penetrates only the areas of the surface layer that have been rendered transparent by the previous imagewise exposure, resulting in imagewise anchorage of the photohardenable layer to the base layer. The remaining photohardenable material, along with the marking layer, is then removed.
Any of the foregoing types of plate can be secured to the plate cylinder of a lithographic press for direct, on-press imaging, after which printing may commence. This configuration requires mechanical clamping mechanisms, and inevitably results in an angular "void" segment occupying the space between the top and bottom margins of the plate. The void prevents printing of a continuous, unbroken image along a web or strip of material, as is necessary for the production of decorative items such as wallpaper. Furthermore, the existence of this segment presupposes precise alignment and control assemblies to ensure proper registration of the plate image with the margins of the substrate to be printed.
The need for elaborate attachment measures arises from the traditional methods of imaging lithographic plates. These tend to be fabricated on graphic-arts production equipment, utilizing coaters and other application devices that operate most readily on flat sheets, after which the image is applied photographically. Imposing a photographic image onto a receptor ordinarily requires a flat receptor surface, as does the succeeding chemical development.
Once the impressions on an imaged lithographic plate have become worn or indistinct, the plate can no longer be used and is discarded. This practice can have unfortunate environmental and economic consequences, particularly in the case of plates that include hazardous materials. Recycling is expensive because of the difficulty of separating and recovering the different plate constituents; the plate must, in general be reconstructed entirely.